An investigation of the presence of ultramicrocells in natural mineral water

The presence of ‘ultramicrocells’ in natural mineral water, capable of passing through a 0·2 μm filter, has been demonstrated. Filters allowing the greatest proportion of viable (culturable) cells to pass ranked in the order, 0·4 μm polycarbonate (5·02%) > 0·2 μm polycarbonate (0·02%) Ð0·45 μm cellulose nitrate (0·02%) > 0·2 μm cellulose acetate (< 0·002%). Following incubation for 4d at 22 °C, viable counts in filtered mineral water increased from < 2–8·7 × 102 cfu ml−1–2·8 × 104–1·9 × 106 cfu ml−1. Successive filtration/incubation cycles of mineral water increased the proportion of cells passing through a 0·2 μm cellulose acetate filter from < 0·003% to 0·11% and 0·69%, suggesting selection for ‘ultramicrocells’. Cells isolated from this process and grown on liquid R2A medium were thin, Gram-negative rods, of 0·15–0·40 μm wide and 0·50–6·20 μm long. Membrane filtration techniques used for pathogen detection in mineral waters will not retain all the cells present. If pathogens are able to form ultramicrocells, these may go undetected.